1. Field of the Invention
This invention relates to improved joints for connecting carbon electrodes to provide electrode columns for use in electric furnaces or like equipment, and to nipples for forming such joints, the term "carbon" as used herein being generally inclusive of the graphite form of carbon.
Conventionally, an electrode joint comprises two coaxially aligned carbon electrode sections having threaded sockets in their butt faces and a correspondingly threaded nipple joining the electrode sections in abutting relationship. Such joint permits continuous electrical operation of an apparatus utilizing electrodes by providing means for attaching new electrode material to the end of an electrode column as the column is consumed.
The thread depth of the nipple utilized in this joint is generally uniform over its entire length, with each thread having a load-bearing flank that carries the force between the nipple and the electrode sections, and a non-load bearing flank on the opposite side thereof. The non-load bearing flank of the nipple is the thread flank opposite the flank which is closer to the plane defined by the abutting electrode sections.
A serious problem encountered with conventional electrode joints is that they are susceptible to failure during use, this failure resulting from breakage due to splits and cracks produced by stresses caused by flexural strain and thermal gradients experienced by the joint when it is being fed into an operating electric furnace. The difference between the coefficient of thermal expansion of the electrode sections and the nipple also leads to stresses within the electrode joint. Further, the high current loading of modern electric furnaces frequently causes additional joint stresses due to overheating of the nipple portion at the junction between the nipple and the electrode sockets.
It is known that failures of electrode joints, which result from the stresses are strains discussed above, are principally localized at or near the first engaged nipple threads near midlength of the nipple or at the first engaged socket threads in the base of the socket.
2. Description of the Prior Art
It has long been recognized that a modification of the conventional electrode joint design was necessary to eliminate failure due to breakage during use. Various methods have been proposed by those skilled in the art to prevent failure by alteration of the joint at or near the midlength of the nipple.
U.S. Pat. No. 3,612,586-Stieber et al. provides a modification for an electrode joint comprising: (a) inactivation of the threads of the socket of the upper electrode section directly adjacent the joint face; (b) inactivation of the nipple threads directly adjacent the upper electrode section end face; or (c) both of these. This reference, while providing some reduction of the stresses at the area of the electrode joint at the first engaged nipple threads at the midpoint of the nipple, does not provide means for distributing the stresses amongst the engaged threads of the joint near this location.
The present invention provides means for modifying the nipple of an electrode joint near the midpoint thereof so as to equally distribute thermally and mechanically induced pressures, thus equalizing or distributing the failure-causing stresses and strains over a large portion of the nipple, thereby providing increased electrode-nipple joint strength.